The increasing complexity and density of circuitry on a printed circuit board and of components on the circuit board increases the difficulty of diagnosing, servicing and repairing improperly functioning printed circuit boards. Even with stringent quality control measures, circuit components have a certain probability of failure. Pretesting components increases product yield, but various circuit components are difficult to test prior to electrical connection to a printed circuit board. For example, an individual integrated circuit chip may have hundreds of closely spaced contact pads, rendering pretesting an arduous task.
One interconnection technique which permits pretesting of chips is referred to as "tape automated bonding." This fabrication procedure utilizes a continuous insulated tape which is similar to photographic film to provide a planar substrate for chips that are attached to individual sections, or frames, of the tape. A spider-like metal pattern of conductive traces is etched on each frame. The traces may either "fan out", i.e. radiate from the center of the frame to the four edges, or may consist of four sets of parallel lines, with each set extending perpendicularly from one edge of the chip. The chip is carefully aligned over the center of the frame so that the contact pads of the chip are precisely located at corresponding conductive traces in the central portion of the frame. The chip is then attached to the tape automated bonding frame. This connection of the chip contact pads to the inner portion of the frame is referred to as "inner lead bonding."
After the inner lead bonding has been performed, the integrated circuit chip may be tested. The chip is thoroughly exercised electrically. The outer lead ends of the frame of a properly functioning chip are typically microbonded to pads on a substrate, such as a printed circuit board. The attachment of the conductive traces of the frame to the pads of the substrate is referred to as "outer lead bonding."
Proper alignment of the tape automated bonding frame with the substrate during outer lead bonding is critical. The conductive traces of the frame are closely spaced. A center-to-center distance, or "pitch", between conductive traces may be 4 mils. Fineline tape automated bonding frames increase the chances of electrical short circuits and of electrical failures resulting from misalignment.
In addition to achievement of precise alignment, planarity is an important consideration in outer lead bonding. The thinness of the leads of fineline tape automated bonding frames makes it more difficult to achieve the necessary flatness. U.S. Pat. No. 4,865,193 to Shimamoto et al. teaches use of reinforcing members on a tape automated bonding frame to reduce stresses exerted on the conductive traces during molding of the chips, thereby minimizing deformations of the conductive leads.
The requirement of planarity of the conductive traces of the frame and planarity of the pads of the substrate becomes more stringent with use of the interconnect system referred to as demountable tape automated bonding. In this system, outer lead bonding is not a microbonding technique, but rather is provided by elastomeric pressure of the outer lead ends onto the pads of the substrate. An advantage of this interconnect system is that a defective chip and its tape automated bonding frame can be easily replaced. However, the interconnect system suffers from two difficulties. Firstly, relaxation of the elastomeric pressure over time and degradation of the elastomer due to temperature variation jeopardize the electrical contact at the outer lead ends. Secondly, in high frequency applications the tape automated bonding frame includes a ground plane on a side of the insulated tape opposite to the conductive leads. This two-metal frame is very rigid and will not easily conform to the pads of the substrate. Such conformity is important in demountable tape automated bonding, but is difficult to achieve with use of elastomeric pressure.
An object of the present invention is to provide an apparatus and method which facilitates repair and replacement of a tape automated bonding frame and which insures conformity of the frame with contact areas of a substrate to which the frame is electrically connected.